The invention relates to a laser Doppler anemometer with at least two laser beams, which are generated by a laser light source and which pass in common to a region of a fluid flow containing particles. Light scattered from the region is measured wherein the frequency of the light is shifted by the Doppler frequency as a result of fluid flow speed.
Such laser Doppler anemometers have been known for a relatively long time. They are employed for the virtually reaction-free measurement of flow speeds of fluids which contain particles. In the region of the flow, the two beams of the laser light source generate a virtual interference pattern, through which the particles of the fluid pass. The frequency of the scattered light produced thereby is shifted by the Doppler frequency in relation to the output frequency of the laser light source.
Continuously radiating laser light sources have customarily been employed for the construction of laser Doppler anemometers (LDA). To the extent that pulsed lasers, e.g., pulsed laser diodes (cf U.S. Pat. No. 4,036,557) have been employed, the pulse duration has been selected so as to be substantially greater than the "transit time", i.e., the time which a particle requires in order to traverse the measurement region.
Continuously radiating lasers which have a high output power for the generation of a good signal-to-noise ratio for the measurement signals are relatively voluminous systems, so that a measurement configuration which is costly and difficult to handle is required. Continuously radiating laser diodes, which have been used for laser anemometry (DE-OS No. 3,435,423), have a relatively weak output power, so that the signal-to-noise ratio of the measurement signals permits the use of the corresponding system, without costly supplementary measures, only for specific fields of application. The same applies, in principle, to the hitherto proposed pulsed laser diodes; in this case, the evaluation of their measurement signals led to considerable problems.